This project is a systematic study of 2 radiobiological phenomena of particular importance in tumors, namely reoxygenation and reassortment. The EMT6 tumor system in BALB/c mice will be used most extensively, and data obtained with this system will hopefully be supplemented with other tumor systems. Having established the kinetics of reoxygenation after single conditioning doses of 1,000 and 2,000 rads, using the paired survival curve method, further experiments will explore the kinetics of reoxygenation after different schedules of fractionated conditioning doses. The reassortment experiments will be designed to investigate the possible changes in the ratio of cycling (P) to non-cycling (Q) cells in this EMT6 tumor system. Primary emphasis will be given to experiments utilizing autoradiography after 3HTdR infusion, to disclose alterations in the percent of cells in cycle. The ability of radiation and chemotherapy to cause such alterations will be investigated, using single divided doses of individual modalities and selected combinations will be investigated, using single and divided doses of individual modalities and selected combinations of modalities. Two other methods based upon the incorporation of the TdR analog, 5-bromodeoxyuridine, into the DNA of cycling cells will be tested: 1. The number and proportion of non-cycling cells will be determined after exposure of cell suspension in vitro to visible light which kills BrU-labeled cells. 2. The feasibility of discriminating BrU-labeled (P) form unlabeled (Q) cells by the reduced fluorescence observed in labeled cells which are first stained with the dye Hoechst 33258. Additional experiments will be done to determine the optimum timing of therapeutic regimens which combine radiation with one or more chemotherapeutic agents.